A multi-mode optical fiber using a plurality of propagation modes has been proposed as a technique to increase the transmission capacity. In particular, mode multiplexed transmission using a plurality of propagation modes is attracting attention as a new large capacity transmission scheme because the transmission capacity can be improved by several times the number of modes.
In transmission using the multi-mode optical fiber, inter-mode crosstalk occurs in the transmission line, so a multiple input multiple output (MIMO) equalizer is used at the receiving end as compensation means.
However, in a case where there is a mode dependent loss (MDL), even if a MIMO equalizer is used, performance degradation of a transmission system becomes a problem (for example, see Non-Patent Document 1). Further, if group delay spread (GDS), which is the spread of pulses related to MIMO signal processing, is large at the receiving end, the load of digital signal processing (DSP) related to MIMO increases, so the DSP load needs to be reduced in order to realize long distance transmission (see, for example, Non-Patent Document 2).
Therefore, in order to alleviate the influence of MDL and DMD, use of a mode scrambler that are configured to generate coupling between modes has been proposed (see, for example, Non-Patent Document 3). A ring core type fiber has been proposed in order to positively generate coupling between modes in an optical fiber transmission line (see, for example, Non-Patent Document 4). Furthermore, a DMD reduction effect has been experimentally checked by generating mode coupling in 2 km of six-mode fiber using a mode multiplexer/demultiplexer (see, for example, Non-Patent Document 5).